Operation of lean burn engines, for example, diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy and have low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Additionally, diesel engines offer significant advantages over gasoline (spark ignition) engines in terms of their fuel economy, durability, and their ability to generate high torque at low speed.
From the standpoint of emissions, however, diesel engines present problems more severe than their spark-ignition counterparts. Because diesel engine exhaust gas is a heterogeneous mixture, emission problems relate to particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO).
NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others. NO is of concern because it is believed to undergo a process known as photo-chemical smog formation, through a series of reactions in the presence of sunlight and hydrocarbons, and is a significant contributor to acid rain. NO2, on the other hand, has a high potential as an oxidant and is a strong lung irritant.
Effective abatement of NOx from lean burn engines is difficult to achieve because high NOx conversion rates typically require reductant-rich conditions. Conversion of the NOx component of exhaust streams to innocuous components generally requires specialized NOx abatement strategies for operation under fuel lean conditions.
One such strategy for the abatement of NOx in the exhaust stream from lean burn engines uses NOx storage reduction (NSR) catalysts, which are also known as “lean NOx trap (LNT).” The lean NOx trap technology can involve the catalytic oxidation of NO to NO2 by catalytic metal components effective for such oxidation, such as precious metals. However, in the lean NOx trap, the formation of NO2 is followed by the formation of a nitrate when the NO2 is adsorbed onto the catalyst surface. The NO2 is thus “trapped”, i.e., stored, on the catalyst surface in the nitrate form and subsequently decomposed by periodically operating the system under fuel-rich combustion conditions that effect a reduction of the released NOx (nitrate) to N2.
Oxidation catalysts comprising a precious metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines to convert both hydrocarbon and carbon monoxide gaseous pollutants by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts have been generally contained in units called diesel oxidation catalysts (DOC), which are placed in the exhaust flow path from a diesel-powered engine to treat the exhaust before it vents to the atmosphere. Typically, the diesel oxidation catalysts are formed on ceramic or metallic substrate carriers (such as, e.g. a flow-through monolith carrier), upon which one or more catalyst coating compositions are deposited. In addition to the conversions of gaseous HC, CO, and the soluble organic fraction (SOF) of particulate matter, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) promote the oxidation of nitric oxide (NO) to NO2.
Catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation because the engine exhaust is not at a temperature sufficiently high enough for efficient catalytic conversion of noxious component in the exhaust.
Oxidation catalysts comprising a platinum group metal (PGM) dispersed on a refractory metal oxide support are known for use in treating exhaust gas emissions from diesel engines. Platinum Pt is an effective metal for oxidizing CO and HC in a DOC after high temperature aging under lean conditions and in the presence of fuel sulfur. On the other hand, palladium Pd rich diesel oxidation catalysts typically show higher light-off temperatures for oxidation of CO and HC, especially when used to treat exhaust containing high levels of sulfur (from high sulfur containing fuels) or when used with HC storage materials. “Light-off” temperature for a specific component is the temperature at which 50% of that component reacts. Pd-containing DOCs may poison the activity of Pt to convert HCs and/or oxidize NOx and may also make the catalyst more susceptible to sulfur poisoning. These characteristics have typically prevented the use of Pd-rich oxidation catalysts in lean burn operations, especially for light duty diesel application where engine temperatures remain below 250° C. for most driving conditions.
Oxidation catalysts with high levels of platinum content cause high conversion rates in diesel exhaust gases in the oxidation of NO to form NO2. Oxidation catalysts which have a large amount of palladium can provide nearly complete conversion of high quantities of unburned hydrocarbons in the diesel exhaust gas even at low temperatures. However, aged catalysts with high levels of platinum content have the tendency to quench in the event of high levels of hydrocarbon content, while palladium does not have a sufficient level of NO oxidation activity. Thus, there is a conflict between the NO conversion performance and colder temperature performance. For cost reasons, this conflict cannot be resolved by means of the addition of two noble metals palladium and platinum in the oxidation catalyst. Moreover, platinum and palladium can interact negatively when combined, such that the additive effect is actually lost. Thus, a diesel oxidation catalyst is needed that resolves such conflict. The NO conversion to NO2 can impact the downstream SCR reaction, especially the “fast” SCR reaction, as described below.
As emissions regulations become more stringent, there is a continuing need to develop diesel oxidation catalysts systems that provide improved performance, for example, lower light-off temperature for fuel used in active regeneration of the a downstream diesel particulate filter. There is also a need to utilize components of DOCs, for example, Pd, as effectively as possible.